Compact exhaust-gas turbochargers generally have exhaust-gas turbines with a strictly radial (radial turbine) or angled (mixed-flow turbine) inlet flow. The exhaust-gas flow is deflected through the turbine wheel, and flows away in the axial direction.
The turbine wheels of radial and mixed-flow turbines are often provided with scalloping. The scalloping refers to a cutout in the rear wall of the hub of the turbine wheel, between the individual rotor blades. The scalloping serves mainly for reducing the moment of inertia by cutting out material in the radially outermost area of the turbine wheel.
According to U.S. Pat. No. 4,659,288 the scalloping contour may be symmetrical with respect to the exhaust-gas inlet edge of the individual rotor blades of the turbine wheel. The scalloping contour runs to a point, or is rounded off, towards the exhaust-gas inlet edge. The scalloping contour is generally likewise rounded off at the radially innermost point on the scalloping contour, that is to say at the lowest point of the cutout in the rear wall of the hub of the turbine wheel, thus resulting in a scalloping contour with a continuous profile from the exhaust-gas inlet edge to the exhaust-gas inlet edge of adjacent rotor blades.
Alternatively, as described in EP 1 462 607 A1 for example, the scalloping contour may have an asymmetric profile between the exhaust-gas inlet edges of adjacent rotor blades.
Particularly in the case of mixed-flow turbines such as those used in the TPS . . . D/E series of ABB exhaust-gas turbochargers, the rotor blades have three-dimensional curvature. On the one hand, each hub/blade junction, that is to say the junction between the rotor blade and the hub, has a curved profile with respect to the radial line. On the other hand, the hub is inclined backwards, toward the turbine shaft, in the area opposite the radially outermost edge. The three-dimensional blade shape can result in asymmetric deformation in the area of the scalloping, at a high rotational speed and when the turbine wheel is thermally loaded. If the scalloping contour is symmetrical, as shown in the illustration in FIG. 2, the rear wall of the hub is drawn radially outwards by the powerful centrifugal forces. The surface on the pressure side of the rotor blade in particular is rotated about the foot of the rotor blade, as is indicated by the thick arrow in the figure. This results in high stresses in the area of the scalloping contour, in particular at the lowest point and, in the extreme, these can restrict the life of the turbine wheel.